Multiplex telegraphy.



F. K. VREELAND. IVIULTIPLEX TELEGRAPHY.

APPLICATION FILED MAYZO, I909.

Patented May l, 19 17.

1m) ntor FREDERICK K. VREELA-ND, OF MONTCLAIR, NEW JERSEY, ASSIGNOR TO VREELAND APPARATUS COMPANY, INC.,0F NEW YORK, N. Y., A CORPORATION OF NEW YORK.

MULTIPLEX IIPELEGRAIIE'HY.

To all whom it may concern:

Be it known that I, FREDERICK KJVREE- LAND, a citizen of the United States, residing in Montclair, county of Emex, State of New Jersey, have invented a certain new and useful Improvement in Multiplex Telegraphy, of which the following is a specification. 5

The invention herein described relates to a system of multiplex telegraphy by means of alternating currents and employing electrically tuned receiving circuits, whereby a number of messages-may be sent simultaneously in either direction over the same line without mutual interference. In carrying out the invention I employ alternating currents of different frequencies, one frequency for each set of sending and receiving apparatus.

A variety of methods have been proposed for multiplex telegraphy by means of alter- .nating currents. and electrically tuned receiving circuits, but these have" failed when applied to lines of practical length or when the number of circuits is considerable.

By the invention herein described it is possible to send a large number'of messages simultaneously over a long line, in either direction, without mutual interference, and it is further possible for any receiving operator to cut in or interrupt the transmitting operator who is sending to him without interfering with the other transmitters or receivers at either end.

Among the factors which have made former systems'impracticable was the lack of suitable means for generating the alternating currents. These currents should be of very constant frequency and pure sine wave form, and it should be possible to vary the frequency to suit varying conditions, All these features are embodied in the present invention by the use of an undamped oscillator, such as those described in my United States Patents Nos. 829,447 and 829,934, and

applications for patents Serial No. 348,085 filed Dec. 15, 1906 and Serial No. 348,403 filed- Dec. 18, 1906, which produce sinusoidal currents of great purity and of extremely constant frequency. A further advantage Specification of Letters Patent.

Patented May 11, 1am.

Application filed May 20, 1909. Serial No. 497,194.

of this form of generator lies in the fact that the frequenciespf the different generators are not bound together by any definite mutual relation, or dependent upon the speed of moving parts as when mechanical generators are used, but each frequency may- Thus mitters are operated in series, each through a separate transformer. Two or three such transmitters may operate together Without interference, but where the number is increased, their mutual action becomes serious and often the combined ,impedance is so great as to interfere with the transmission of signals. where the transmitters are operated in parallel and where their circuits are associated so as to mutually affect each other.

In the present invention, this mutual action is eliminated by the use of a plurality of sifting circuits of special construction associated in parallel relation with a common conductor or line, each tuned to the frequency of the generator or oscillator to which it is coupled and operating independently of the other circuits.

Another cause of failure in previously proposed systems is dueto a misconception as to the character of the received energy which constitutes the signal impulses. A. line of considerable length does not act as A similar disadvantage occurs a rigid coupling between the sending and receiving apparatus, across which the signals are transmitted by main force, as it were; it acts rather as a yielding connection possessing both elasticity and inertia, whose capacity and inductance take up energy from the transmitter and pass it on from section to section as the signal traverses the ceiver.

The receiving apparatus must be capable of taking up and absorbing these transitory energy pulses without reflection while discriminating between impulses of different frequencies. If the receiving system possessesconsiderable impedance the waves will be in large measure reflected and the signals will be lost.

The methods which have been heretofore proposed may be divided broadly into two classes: First,those in" which the difierent sets of receiving apparatus are connected in series; and, second, those in which they are connected in parallel across the vline. In the former class the impedance of all the receivers acts cumulatively to the detriment of each individual and even though the im pedance'of the individual units is small, the aggregate of a large number is so large that 26 the bulk of the wave energy will be reflected. When the receivers are connected in parallel this cumulative impedance is avoided, but the selectivity of the receivers is poor because of the detrimental influence of the 30 line losses on the selectivity of the receiving circuits. A receiver of the former type may be sharply selective and one of the latter type may be non-reflecting; but these two important functions are not combined in any system that has been heretofore-proposed.

Inthe system herein set forth both these functions are combined to the fullest extent. Each receiving system which is shunted across the li'neis rendered non-reflect- I 40 ing by the use of a sifting circuit, of practically negligible impedance at the frequency for which it is tuned, so that impulses of this frequency pass through it'- without material loss, and having an un- '45 balanced reactance at the nearest foreign frequency which is large withrelation to the equivalent impedance of'the line'so that the loss by leakage through other sifting circuits than-the one designed to select the particular frequency will be small. By giving the sifting circuits this large unbalanced reactance they become roughly or moderately selective, notwithstanding their intimate tion: Fig. 2, shows in more detailed form the arrangement of the 'diflerent parts of one such apparatus: Fig. 3 is a view in section of a form of telephonic receiver peculiarly adapted for use in my system.

Referring particularly to Fig. 1, this figurerepresents, for the sake of simplicity, only three combined sending and receiving sets at each end of the line. Each of these sets, 5, 6, 7, 8, 9, and 10, comprises a sending apparatus and a receiving apparatus, which are adjusted to operate on the same frequency as the corresponding set at the opposite end of the line. In each of these combined sending and receiving sets, 0 is an undamped oscillator or its equivalent which generates an alternating current of pure sine wave form and constant frequency.

The form of oscillator illustrated at sets 5,

6, 8 and 10 is that described in my patents before referred to. In the form illustrated, the oscillator comprises a mercury vapor tube 16 having anodes 17, 18. and cathode 19. Power is supplied to the tube by the battery 20, or other so urce of current, which is connected between the cathode 19 and the anodes 17, 18, through choke. coils 21,. 22. The

oscillating circuits 17, L C P 18 is connected across the anodes. Energy derived from the source 20 is converted in the tube 16, into a succession of impulses, which are applied to the oscillating circuit alternately through the anodes'17 and 18. The inductance coils L P of the oscillating circuit, .when energiaed' by: these alternating im-' pulses, produ e an alternating magntic field which tends to deflect the discharge in the tube, directing it first toward one anode and then toward the other. The frequency of the energy impulses produced by the energy supplying element, which is otherwise indeterminate is automatically determined by the natural period of the oscillating circuit and thus maintained constant. The oscillator feeds current into the line through a sifting circuit S C P comprising a large inductance S P and a small capacity C the capacity and inductance being so proportioned that the natural frequency of the circuit when bonnected to the line is the same as that of its oscillator. The oscillator may be directly coupled inductively to this siftingcircuit, as is indicated in set 6, but I v tems of' which these sifting circuits form a a receiving, circuit of high eificiency.loosely eoi'coupledto'j-gthe sifting'eircuit, an effective 1 ed' therewith several sets of transmitting and apparatus embodymg my invenconnection with the line whose energy losses prefer to use an intermediate coupling cir- 55 are conslderable :The severalreoe1v1ng sys-.

part are made'highly selective by associating with. each sifting'circuit a tuned resonant mechanical,resonatonor'both. f 1

1 1- he .dr' wing, Figure'l is a diagram- ,m'atic view of a telegraph line having connectconstants of the coupling circuit, as by shunting the whole ora part of the inductance P by a key la or introducing additional capacity by a key I), (set 5). The coupling circuit is not necessarily a tuned circuit. It may be without capacity and consist only of the coils S and P and key K (set 8). The receiving apparatus comprises preferably a resonant receiving circuit, S L (set 7) which is coupled to a portion of the inductance P of the sifting circuit. This resonant circuit is also tuned to the natural fre quency of the signals which it is to receive, and it includes the receiving or translating device R.

The oscillator O generates current of'a definite frequency depending upon the capacity C and the inductance L P of its oscillating circuit. This. current may be communicated to the sifting circuit S C P directly through a transformer or equivalent coupling device P S as shown in set 6, in which case the signals may be produced by opening and closing the circuit of the oscillator (set 6) or by 0 ening or closing the sifting circuit (set 10) I The various sifting circuits are connected in parallel across the line. Their function is, first, to couple each transmitter and its 30. corresponding receiver to the line, and second, to perform the first rough sorting or sifting of the signals before they are presented to the sharply selective resonant receivingcircuits. They are so designed that each will permit current of its own frequency to flow freely from its oscillator to the line and will also permit received signals of the same frequency to flow freely through it, thus energizing the receiving circuit coupled thereto. Foreign currents of other frequencies, however, whether from the home .end of the line or from the distant end, will be practically excluded, provided the circuitis designed as hereafter specified.

The resonant receiving circuit S L (1 R (set 7) should be a very efficient resonator, that is it shouldpossess as small a damping factor as possible, and it is preferably loosely coupled to the sifting circuit 2'. e.,-the mutual inductance of the two circuits should be small compared to their self-inductances;

thus the primary coil P or-the secondary coil S or both, should havesmall-inductance compared to the total inductance of their respective circuits, or the two coils should be loosely interlinked. The receiving circuit will then be sharply selective and will not respond to any small currents of foreign frequencies which may leak through the sifting circuit. *It will, however, respond both to the signals sent out by its own transmitter and to those received from the corresponding transmitter at the other end of the line. The sending operator thus hears his own signals, and when the receiving line a current which interferes with the sig-.

nals heard by the sending operator in his receiving circuit.

In certain cases it is desirable to insert the sending key in the sifting circuit as shown in the sets 7 and IQ. In this case, the receiving operator keeps his key (or the switch shunting it) closed and-the sending operator produces his signals by opening and closing his sifting circuit. When the receiving operator desires to cut in or interrupt the sender he opens his key, thus interrupting the sifting circuit. This is practically equivalent to opening the line at the receiving end, as the other sifting circuits, tuned to different frequencies, will not permit the passage of any considerable current at the frequency which is being used. There will, however, always be some current flowing through the sifting circuit at the s nding end, due partly to the imperfect seiectivity of the other sifting circuits, and partly to the leakage and capacity currents in the line. Hence, in order that this method may be effective, all of these shunting agencies should be reduced to a minimum. If they are reduced to a suflicient extent, signals may be sent over the line (as from set 7) by opening and closing the sifting circuit, using the oscillator at the receiving end (set 10) to supply the current. If, however, the line and other losses are too great. the residual current in the receiving circuit of set 10 when the sifting circuit of set 7 is opened will be too great to give clean and effective signals.

In Fig. 1, each of the combined sending and receiving sets illustrates a difi'erent arrangement in detail, although they all embody the more important elements of the invention. In set 5, the oscillator is shown coupled to the sifting circuit through a tuned coupling circuit, S C P K. In set 8, the coupling circuit is not timed, the con denser ,0, being omitted. In set 6. the coupling circuit is omitted altogether, and the oscillator is coupled directly to the sifting circuit through the mutual action of the coils P and S In this set, the key K is shown inserted in the oscillating circuit of the oscillator. In set 10, a similar coupling is used, but the sending key is located in the sifting circuit. In set 9, the undamped oscillator is replaced by any suitable generator of sifting circuit in two parts which are relatively movable so that their combined inductance' may be varied for purposes of tuning. The condenser C is conveniently located between these two sections. The inductanc'e may be all concentrated in one coil, however, as shown in set 9. It is often desirable, where the inductance of the sifting circuit is very large, to wind the coils in pancake form, or with a wedge-shaped crosssection so that the self induction will increase gradually from the terminals which are connected to the line to the center of the circuit. Such a construction has the further advantage of reducing the distributed capacity of the coils. This graduation is shown at set 6. It is sometimes desirable also to divide the self induction into more than two sections which are separated by condensers. Thus at set 10 the inductance of the sifting circuit is shown as in four separate sections, L S P, L,, separated by condensers C This arrangement has the advantage of diminishing the potential differences between different parts of the circuit, which may amount to several hundred volts even when the current is not more than 10 milli-amperes, because of the large self induction and small capacity of the circuit.

The six sets shown in Fig. 1 differ also in regard to the method of coupling the resonant receiving circuit to the sifting circuit.

In set 5, the coupling is a simple inductive one. The fact that the coupling is loose is' indicated by the relatively small number of turns in the secondary coil, 8,, and by the 'fact that it is in inductive relation with only a portion of the inductance of the sifting circuit. Inthis set, the coil, L constitutes the bulk of the inductance of the resonant circuit. In set 6 also the coupling is inductive, but the primary coil, P is a separate coil from those which constitute the bulk of the inductance S, L of the sifting circuit. This coupling also is a loose'one, by virtue ing them that they are-feebly' interlinked,

' for example, by separatingthem or by plactrical connection, a portion, S of the coil P,

ing them with their axes nearly at right angles. In set 8 the 'couplingis a direct elecof the sifting circuit being made also 'a part of the resonant circuit. The looseness of the coupling here is indicated by the fact that this portion S; is small compared with the coils ,1 and L In set 9 the coupling is accom' lished through the agency of a con-' denser, across which. the resonant-circuit is connected. If the capacity of this condenser is large compared to that of the condensers C and C the coupling will be a loose one.

Various arrangements of the receiving device R are also shown. In set 5, the receiver is an electrostatic telephone or,other electrostatic device which posseses a capacity of its own, and the separate condenser C in the resonant circuit is not required. In set 6, the electrostatic receiver is provided with the amplifying'device described in my application Serial No. 350,476, filedJanuary 2, 1907, which comprises a battery B by which the receiver R is excited, or charged to a potential difference which is usually high compared to the potential differences caused by the signal impulses. A large condenser, 0.,

is connected in series in'the resonant circuit and performs the double function of preventing a short circuit of the exciting battery and coupling thisbattery to the circuit without interference with the oscillations.

A resistance is preferably inserted in the battery circuit, but this is not required nor is an inductance necessary provided the capacity of the condenser C is large with respect to that of the receiver R. In set 7, an additional condenser C is shown in parallel with the receiver R. .This arrangement is required when the receiver has not of itself sufiicient capacity for tuning. This arrangement may also be used with receivers of other than the electrostatic type, provided they possess sufficient impedance to prevent undue damping of the resonant circuit. In set 8, the receiver R is shown connected in series in the resonant circuit. It may be any suitable receiver or translating device in which the resistance and other energy losses are small. For example, it may be a magnetic telephone as illustrated or other magnetic device, in which case its own self-induction may be suflicient to constitute. the whole or a part of the tuning inductance of the resonant circuit. In set 10, It" is a relay, preferably of the vibratory type, which is capable of responding to the alternating signal currents and which closes a local circuit including a translating device T. In set 9, the receiver R is located in the secondary' circuit of a transformer P S whose primary is in series in the resonant circuit.

The proper proportioning of the sifting circuit 1, C, S, is a matter of great im-' portance. v It is not sufficient simply to tune this circuit to the proper frequency, but its selfinduction and capacity must also be suitably proportioned with regard to the line impedance, i. e., the self induction must be so large and the it i o small that;

mamas capacity reactance is i where L is the inductance and C the capacity of the circuit. When the circuit is tuned to the frequency N the capacity reactance is equal and opposite to the self induction reactance, and therefore For any other frequency N the self induction reactance will beLo) and the capacity i hence the unbalanced rereactance actance of the circuit will be In order that one 'sifting circuit tuned to the frequency u) may feed current through the line without robbing the line of any considerable current at the frequency w fed into it by another sifting circuit, this unbalanced reactance, it, must be large compared with the equivalent impedance of the A numerical example will illustrate: Suppose the frequency N tobe 500 cycles per second, and the inductance L to be .25 henry.

The self induction reactance, Lo), will then be 21c 500 .25:7 85 ohms. The capacity reactance, 3 is'also equal to 785 ohms; hence, the capacity will be 1 -o 21rX500X785 farad or .41 microfarad. If the next adjacent frequency N is 5 per cent. higher, that is 525 cycles, the self induction reactance for currents of this frequency will then be 824 ohms and the capacity reactance 7 46 ohms. The total unbalanced reactance will then be the difference between these two or 78 ohms, which is a figure too small to make such a circuit operative over lines of practical length. Hence it is important to make the self induction very much larger than the value just assumed and the capacity correspondingly small. Thus, if we make the self induction 40 henries, the capacity will then be about .0025 microfarad, the self induction reactance at frequency N will be 160 times as great, that is 125,600 ohms, and

the capacity reactance also 125,600 ohms, so

that the total reactance at this frequency N is zero. At the frequency N, however, which is 5 per cent. greater or 525 cycles, the self induction reactance becomes 131,800 ohms, and the capacity reactance 119,200 ohms, and the difference, 12,600 ohms, is the total unbalanced reactance at the frequency N. lhis is large enough to enable the ap paratus to workover lines of practical and even considerable length. Thus, if the equivalent impedance of the line be 2,000 ohms, the amount of current at frequency N which will be shunted through this sifting circuit will "be (roughly) or. about 16 per cent. of the current which flows out of the line at the'distant end. This amount of foreign current is usually permissible in practice, but it may be reduced by still further increasing the inductance and diminishing the capacity. Where the current delivered at the receiving end is much smaller than that fed into the line, due allowance should be made for this difference in determining what unbalanced reactance is required in the sifting circuit. The largest I foreign current in the sifting circuit must be sufliciently small in relation to the received signal current to come within the range of selectivity of the receiving apparatus. I Thus the term equivalent impedance, as here used, includes not only thequantity ordinarily known as the line impedance, which depends only upon the line constants per unit length and-not upon the length of the line, but includes also the line attenuation, which is a function of the length. For longer lines the self induction must be greater and the capacity smaller, or else the difference between the two consecutive. frequencies-will have to be greater.

The frequencies which may be employed in practice covera wide range, but this range is usually limited by certain practical considerations. Thus, if the frequency is made too high, the lineif it is of considerable length or has considerable capacitywill not readily transmit the current. This upper limit of frequency may be considerably raised by the use of loaded lines which are now well known in the art. 'Again, it is undesirable to make the frequency too low because it is convenient to have the note emitted bythe receiver within the range of distinct audibility and to keep the dimensions'of the coils and condensers within convenient limits;v Furthermore the briefest signal impulse must contain a sufficient number of current cycles to act selectively on the receiver. On account of these limitations it becomes necessary, when operating I find it convenient in practice to work with a frequency difference of from five to quency to the natural frequency of the circuit it is necessary to employ a large inductance and small capacity in the sifting circuit when the frequency difference is small. When the inductance is very large and the capacity very small it becomes necessary to consider the distributed capacity of the inductance coil itself, and to so design the coil or coils that this shall not be excessive.

In tuning the sifting circuits due regard should be had for the equivalent capacity or self induction of the line, when these are great enough to affect the natural frequency of the circuits.

It is important also to have the coupling circuit S C P K suitably proportioned with respect to the other elements of the system. This circuit must be capable of transmitting the necessary energy from the oscillator to the line without reacting unduly upon the oscillator. To this end it should preferably be tuned to the same frequency as the oscillator and loosely coupled thereto. Its coupling to the sifting circuit may be closer, but not so close as to alter materially the period of the siftin circuit when the coupling circuit is close The resonant receiving circuit is required to perform the final selection between the signals for whose frequency it is tuned and foreign signals of different frequencies which may leak through the sifting circuit because of its relatively imperfect selectivity. Hence the receiving circuit should be sharply selective'and strongly resonant. To this end, it should be loosely coupled to the sifting circuit and its damping should be reduced to the smallest possible point; in other words, the amount of energy dissipated or transformed into useful signals should be small compared with the total energy stored in the oscillations of this circuit. Thus the resistance of the circuit should be small compared with its reactance and the condenser losses should be as small as possible. The receiver also should be one which does not waste energy in ohmic resist- A receiver which possesses all of these properties in a marked degree is an electrostatic telephone or an equivalent electrostatic motor. One form of electrostatic telephone which is suitable for this purpose is shown in my application Serial No. 350,47 6 filed January 2, 1907, and consists in a flexible diaphragm of conducting material, or of insulation with a conducting surface, mounted in close proximity to a second conductor which is usually fixed. The electrostatic attraction of the two plates causes the diaphragm to vibrate and produce an audible signal. This form of receiver is shown diagrammatically at R in set 7.

Such a receiver is very efficient, the dielectric losses are small and the energy absorbed is very largely converted into mechanical work in the form of vibrations of the diaphragm. It also shares with other forms of receiver embodying a non-reversing electro-mechanical couple operating according to the square law, a certain degree of inherent selectivity for strong rather than weaker signals. The direction of the mechanical force between the two elements of the couple is constant, and does not reverse with the reversal of the current. The magnitude of the force is proportional, not to the current or potential difference, but to the square of the current or potential difierence. The couple operates according to the square law. This fact in itself makes the receiver inherently selective to a certain extent, for the reason that the signals due. to impulses of the proper frequency are proportional to the square of the relatively large current of this frequency, whereas the action due to the smaller currents of foreign frequencies, is proportional to the square of these smaller currents. The foreign actions, therefore, are much smaller in relation to the normal signals than they would be if the indications were proportional simply to'the current. A magnetic telephone in which the usual permanent magnet is replaced'by an electrov magnet, or an ironless coil, traversed by the static telephone is so small as to require a very large self induction in tuning the resonant'circuit. Hence it is often desirable to connect it in parallel with an additional condenser C, as in set 7 orto use the multiple leaf form described later. The sensitiveness of the device may be increased to any desired extent within practicable limits by the use of the amplifying device shown in set 6.

The exciting of the telephone by giving it a large initial charge causes it to be responsive to smaller variations of potential. It should be remembered, however, that in thus increasing the intensity of the signal, a larger amount of energy is translated into mechanical work, and hence the damping of the resonant circuit is increased and its selectivity correspondingly diminished. The selectivity is still further diminished by the fact that the receiver no longer operates according to the square law. By varying the voltage of the exciting battery, B, the sensitiveness of the receiver may be controlled at will and the best possible compromise efi'ected between the conflicting properties of sensitiveness and selectivity. Where the sharpest selectivity is desired, it is usually preferable to employ the amplifying device only to a moderate extent or to omit it altogether, the required intensity of the signal being attained by increasing the total energy stored in the oscillating circuit by means of larger line currents or closer coupling. j

The disk form of electrostatic telephone above described has usually a small capacity. Where a larger capacity is desired in the resonant circuit I prefer to use an electrostatic telephone (see Fig. 3) made up of a number of sheets (Z of tin foil or other conducting material interleaved with thin sheets 6 of mica or other dielectric. The whole is loosely mounted in a receptacle 7 provided with a suitable ear-piece g and a pressure plate it at the back by means of which the leaves may be brought into more or less intimate contact by turning a screw 2'. The-sheets of tin foil are connected alternately to contact sleeves m, n, in which the terminals are inserted. The leaf farthest from the pressure plate is the one whose vibration is communicated to the ear. It is preferably clamped after the. fashion of a telephone diaphragm in order that it may Vibrate ehiciently. Receivers of this type may be made of any desired capacity and so may be used without additional condensers. The capacity may be adjusted over a considerable range by varying the compression of the leaves.

Another desirable feature of the electrostatic telephone when not excited by a local electromotive force is that the note it emits is an octave higher than that corresponding to the frequency of the alternating current. This is an advantage when the frequency is too low to give the most effective audible impulse, say below 500 cycles per second. When the telephone is excited the note-is an octave lower 2'. e., that corresponding to the frequency of the current, which is an advantage when the frequency is above the most effective aildible range. M

The electrostatic telephone, although peculiarly adapted to the purposes of the present invention, is not the only form of re-.

ceiver that may be used. For example, a magnetic telephone may be employed. This may be of the ordinary form, but it is preferably specially constructed with a view to high efiiciency and small iron and resistance losses. It may be connected in series in the resonant circuit as in set 8, orit may be inserted in the secondary'of a transformer P 8,, as in set 9. It is also sometimes desirable to use a relay controlling a local circuit, or operating an additional length of line as in set 10. A number of forms of vibratory relay have been devised and are well known in the art, or a nonvibrating relay may be usede. 9., one in which both fixed and moving elements are excited by the alternating current so that the force does not reverse with the current. The moving element is made slightly sluggish to prevent opening of the circuit during the periods of reversal. 4

Additional sensitiveness may be given to the receiver without a corresponding expenditure of energy by tuning it mechanically to the frequency of the received signals. In the case of the telephonic receiver, this may be accomplished in any of the Wellknown Ways of mechanically tuning such instruments. I

A very effective method of mechanical tuning is the use of a resonant chamber (q, Fig. 3) adjacent to the vibrating diaphragm. Such an acoustic resonator if constructed in the ordinary way is bulky and cumbersome, but a very effective resonator may be made in a very small compass by providing it with an adjustable openin-g t communicating with the outer air. By regulating the size of this opening the tune of the resonant chamber may be adjusted at will. This device per 86 will be made the subject of a separate ap plication for patent.

A convenient arrangement of the component parts of the system is shown in Fig. 2, where a single sending and receiving set is illustrated. O is the oscillator whose oscillating circuit comprises the primary coils I, and condenser C S is a secondary coil which is pivoted for the purpose of varying its inductive relation with the primary. The inductance P of the coupling circuit S, G, P, K is made in two parts which are relatively movable for the purpose of tuning. The condenser G also may be adjustable. The sifting circuit comprises two large coils S, L separated by the condenser C and a third coupling coil, usually small, P Coils S L are relatively movable for ing is performed by adjusting the coni denser C and the fine tuning by shifting the coils.

The. coupling between the coupling circuit and the sifting circuit may be adjusted by changing the relative positions of the coils 1 20 purposes of tuning. Usually the rough tunmary coil P 2 R is the receiver which is also the capacity of the resonant circuit.

The six sending and receiving sets shown in Fig. 1 are not necessarily located at opposite ends of the line only. They may be distributed along the line or on branch lines at any points desired. Each set, comprlsing oscillator, sifting circuit and resonant receiving circuit, is a separate unit and can be located Wherever it is required. Thus a single sending station may call at will any one of a number of stations distributed along the line and each station may communicate at will with any other station if desired. It is also evident that a number of lines may be operated by a single bank of oscillators or equivalent generators. The system is also applicable to so-called phantom lines.

When stations are distributed along a line, and each requires only one circuit the installation is simplified. Usually each station will have its own tune for receiving,

and another station desiring to call it will send on this tune. If the local stations communicate only with a central oiiice, such as a train despatchers oflice, a single complete set will suffice for each local station, while the central station has a bank of oscillators and circuits.

What I claim is:

1. In a system of multiplex signaling, the combination of means for generatin a plurality of alternating currents of diflerent frequencies, means for producing signal impulses by these currents, a plurality of tuned sifting circuits connected in parallel to a common line and having a'large.

unbalanced reactance for small frequency differences relative to the equivalent line impedance, means for impressing the signal impulses upon the line through said sifting circuits, and means for selectively observing at a distance the signal impulses of different frequencies.

2. In a, system of multiplex signaling, the combination of a plurality of undamped oscillators, generating alternating currents of pure sine wave form and of difierent automatically-regulated frequencies, means for producing signal impulses by these currents, a plurality of tuned sifting circuits connected in parallel to a common line and having a large unbalanced reactance for small frequency differences relative to the equivalent line impedance, means for impressing the signal impulses upon the line through said sifting circuits, and means for selectively observing. at a distance the signal impulses of different frequencies.

3. In a system of multiplex signaling, the combinationwith a lineof means for impressing thereon alternating current signal impulses of different frequencies, a plurality of. tuned sifting circuits connected in parallel to said line and having a large unbalanced reactance for small frequency difierences relative to the equivalent line ated with said sifting circuits.

4. In a system of multiplex signaling by alternating currents of different frequens 5. In a system of multiplex signaling by alternating currents of different frequenimpedance, and selective .receivers assoclcies, the combination with a line, of a .plu-' I rality of tuned sifting circuits at eachend of the line, connected in parallel therewith and having a large unbalanced reactance for small frequency difierences relative to the equivalent line impedance, a plurality of undamped oscillators, generating alternating currents of pure sine wave form and of different automatically-regulated frequencies, means for impressing on the line at each end signal impulses of different frequencies from such undamped oscillators, and selective receivers associated with said sifting circuits.

6. In a system of multiplex telegraphy, the combination with a common line, of a plurality of shunted tuned sifting circuits, each having a large unbalanced reactance, relative to the equivalent line impedance, at the nearest foreign frequency.

7. In a system of multiplex telegraphy, the combination with a common line, of

a plurality ofshunted tuned sifting circuits,

10. In the system of multiplex telegraphy by alternating currents, a. tuned sifting circuit having large aggregate self-induction and capacity reactances sub-divided by interspersing a plurality of capacity units between a plurality of inductance units.

5 11. In the system of multiplex telegraphy by alternating currents, a tuned sifting circuit having large aggregate self-induction and capacity reactances sub-divided by interspersing a plurality of capacity units be- 7 tween 'a plurality of inductance units, the 1m reactances of the successive units substantially balancing each other.

This specification signed and witnessed this 19th day of May, 1909.

FREDERICK K. VREELAND. Witnesses: JOHN L. Lorscn, WM. J. CASEY; 

